WHAT: Defining a population is multi-faceted


  • A group of organisms from one species…
    • abundance
    • sex ratios
    • age structure
    • density


  • Defining the population boundary…
    • a lake
    • ecosystem
    • country or state


  • Aphid population: leaf, plant, grassland, ecosystem?
    • depends on the study and the questions

WHY: Understanding population structure (Demography)


  • is population size ↑ ←→ ↓ ?


  • Age of a population, peak reproductive ages


  • Frequency of reproduction, reproductive effort


  • Sex ratios determining mating potential


  • Measuring, predicting and evaluating population sizes guides decisions
    • conservation, pest management, harvesting

Case Study: Deer population size


Case Study: Deer population density matters too…


Case Study: Zombie deer and population density


WHO: Definition of an individual can be tricky



  • Unitary organism: forming a single entity
    • deer, bird, spider, etc.


  • Modular organisms: colonies, etc.
    • bacteria, algae, corals
    • plants: ferns grow by rhizomes


  • Some modular organisms may themselves be populations

HOW: Estimates from representative samples






  • Lessons from Lab:
    • Census techniques
    • Mark & Recapture
    • Spatial component
    • Temporal component



  • Most methods are indirect estimates

Populations dynamics: Movement (dispersal and migration)


Life History: More than just birth and death

Reproductive effort and life cycles differ greatly

Semelparous: One-off reproduction


Species with short life cycles: Annuals & Ephemerals

Life History: Sexual conflict



  • Annuals vs Perennials
    • not just plants







  • When and how to spend your resources
    • reproduction comes at a cost of growth
    • flowers + seeds = ↓ leaves and roots
    • are enough resources available to support pregnancy in animals?
    • surviving vs growing the population…
  • Life histories of males and females may differ

Populations and life cycles: Dormancy


  • Annual organism may spend part of the year dormant
    • Seeds, Spores, Cysts, etc.


  • Thousands of seeds per square meter of soil


  • Dormancy can last for long periods of time
    • protection from harsh environments


  • How does this affect population estimates?
    • Reproductive effort (seeds) and offspring may not be synced















  • Populations are composites of individual Life Histories
    • who survived from previous year
    • new individuals (births/immigration)
    • survivability of offspring


Survirvorship =

Populations dynamics: Monitoring birth and death



  • Measuring rates of birth and death can tell us if a population is growing or shrinking


  • Cohort Life tables: follow the fate of individuals from a single cohort
    • survivorship
    • difficult for mobile animals


  • Static life table: count # of survivors at different ages


  • Tells us a lot about age structure of a population

r/K selection theory: Growth and reproduction strategies





  • r-selected species: “cheap” offspring


  • K-selected species: “expensive” offspring



  • These are the 2 ends of a spectrum

Populations with unlimited resources: Exponential growth

Population growth regulation


  • Density-dependent
    • competition
    • predation
    • disease
    • resources


  • Density-independent
    • weather events
    • disturbances


  • Carrying Capacity:
    • environmental resistance leads to Logistic growth